Prof. McGonagall's biscuits

[cyberflower27]

No: HPFGUIDX 92507

Hi! I'm a newbie from NJ!
I don't know if this has been covered before but here I go: on page 
248 and 249 of OotP (American ed.), while Harry is grumbling to Prof. 
McGonagall about Umbridge, she keeps telling him to "have a biscuit". 
When he says no the second time, she tells him "Don't be ridiculous." 
Why wouldn't she say something like "Suit yourself"? This has 
bothered me since I first read the book. We all know that there are 
so many little clues that are easy to miss throughout the books. It 
almost seems out of place. Almost.
Does anyone have any theory on what the significance of the biscuits 
are or what they may be? I'm pretty sure they're not chocolate...